The invention relates to a method for contacting gases and liquid droplets for mass and/or heat transfer in a spray tower in which liquid is injected at a number of levels in counterflow to the gas, the gas being fed through at least two inlet openings in the shell of the spray tower, and to a corresponding spray tower.
The invention can be applied in spray towers                for mass transfer between gases and liquid (absorption, desorption), for example for absorbing pollutants from exhaust gases, for example for flue gas desulfurization (open spray scrubbers) from acid exhaust gases of combustion processes in the industrial sector, power plants or waste incineration plants, or        for gas conditioning, gas moisture saturation and/or gas cooling, in particular of flue gases.        
What is involved here is a method in which scrubbing liquid or water is injected as droplets into the mostly hot gas stream. The invention can be applied to common flue gas compositions and typical temperatures of around 200° C.
The most used method is the wet cleaning method based on an aqueous limestone-gypsum suspension. A suspension of water, gypsum and limestone is used in this case as scrubbing liquid. The solids concentration of the suspension is 10% by weight, as a rule. It consists predominantly of gypsum and a limestone concentration of between 2-3% by weight in the solid, that serves as absorber. The literature includes an overview of this method from Soud H. N., Takeshita M., FGD handbook, IEA Coal Research, London, 1994. A more up-to-date summary relating to flue gas desulfurization methods is to be found at DTI, Flue gas Desulphurisation (FGD) Technologies, Technology Status Report 012, http://www.dti.gov.uk/ent/coal, 03/2000.
It is customary to use the apparatus concept of the open spray tower for the absorption. In this case, gas is introduced into the spray tower, which has a round cross section according to the latest prior art, in the lower region of the contact zone, and led upward through the scrubbing zone. The contact zone—termed the absorber part in the case of desulfurization—is equipped with spray levels—piping at different heights, at the ends of which are seated spray nozzles—and situated between the bottom surface and uppermost spray level. The scrubbing liquid is injected into the rising gas stream in the form of droplets via various spray levels in counterflow thereto, and collected after the passage of the flue gas in the scrubber bottom situated therebelow. The circulation of the liquid flow is effected in this case via circulating pumps that convey the suspension from the scrubber bottom to the height of the spray levels.
In most spray towers, flue gas is introduced in this case in a lateral and radial fashion through a flue gas duct in the lower region of the contact zone of the absorber. The sole inlet opening has a cross-sectional area such that the inlet speed is in the region of 15 m/s for a maximum flue gas flow.
The liquid is atomized by one-material nozzles, and the majority of the droplets carry out a falling movement in counterflow to the gas until deposition on the scrubber wall or in the bottom.
The interaction resulting therefrom between gas and dispersed liquid results during operation in a multiphase flow that has a decisive effect on the mass and/or heat transfer between the phases. The effect of this in the case of SO2 absorption is to determine the separation efficiency of the pollutant from the flue gas—or the efficiency of the flue gas saturation, for gas conditioning. An important parameter here is the dwell time distribution of the gas in the contact zone. It determines the average contact duration of the gas with the scrubbing liquid.
By contrast with the ideal flow, which is presupposed on designing the method, in the real spray tower there is no uniform upward or axial speed for the gas. That is to say, different axial speeds form in the cross section of the spray tower, and they can deviate significantly from the average speed.
In spray towers of industrial scale, above all, the gas dwell time influences the function of the apparatus. An uneven gas distribution in the contact zone leads to an irregular contact duration between the phases. The effect is a reduced or unbalanced mass transfer in the spray tower cross section that can be found again as a local high SO2 residual concentration in the pure gas in the case of flue gas desulfurization. It leads in the application for flue gas conditioning to the formation of gas strands in the conditioned flue gas that still have an increased temperature. They can damage downstream heat-sensitive apparatuses, or impair their functioning.
The gas dwell time is determined, firstly, by the type of droplet injection. A nonuniform injection with scrubbing liquid leads in the spray tower cross section to a different flow resistance that causes the gas to be deflected outward into regions of less pressure loss. As a result, the interaction with the injected liquid is also less for these partial gas streams.
The way in which the gas is introduced into the contact zone must be regarded as a second important factor. Particularly in the case of scrubbers of large diameter, the requisite transverse movement of the gas has an increasing effect in the contact zone that is necessary for a uniform gas feeding in the spray tower cross section. The ratio between the spray tower diameter D and height of the contact zone H normally varies between D/H=0.40-1.10.
In conventional spray scrubbers, the gas stream is introduced through a rectangular inlet into the spray scrubber with a round base surface. The curvature of the scrubber causes gas layers at the side walls of the gas duct which opens in to be led longer horizontally than those in the middle. Consequently, the gas stream in the middle of the inlet can shift earlier to an upward movement than in the edge zones. The portions of the gas stream at the lateral edge of the inlet advance further into the scrubber and reinforce the effect that is denoted in plant engineering as “edge flow” of the spray tower. What is involved here is the lesser content of scrubbing liquid in the wall zone by virtue of deposition of the droplets from near-wall nozzles on the apparatus wall. The internal region of the scrubber has, by contrast, a higher proportion of the liquid volume phase, since it is possible there for droplets to move longer on a flight path through the contact zone before they are deposited in the scrubber bottom.
In combination with increased gas speeds at the wall of the spray tower, the separation efficiency is perceptibly worsened in these regions, and can be detected in locally increased SO2 residual concentrations in the purified gas. It is even possible in relatively small apparatuses for stagnation point flows to form at the spray tower wall, in which case undesired increased upward gas velocities can arise at the spray tower wall by virtue of the deflection.
Furthermore, the gas flow of the conventional radial inlet induces a compensating eddy in the cross section. The turbulent flow leads to a reduction of the kinetic energy contained in the gas. The dissipation occurring because of the turbulence takes place in a region where the flow resistance owing to liquid droplets is also greatest in the two-phase state. The gas movement is undesirably slowed down in a region in which the gas has already covered a lengthy path through the contact zone. Moreover, the gas experiences an increased resistance there owing to a higher volume phase fraction of droplets, and the tendency of the gas to be deflected outward additionally exists during operation. There necessarily ensues in the horizontal cross section of the spray tower an irregular contact duration with the dispersed scrubbing liquid and the consequences already mentioned for the mass transfer.
Similar problems also arise with the spray tower of DE 100 58 548 C1, where the gas is introduced tangentially into the spray tower through two separate opposite gas ducts. A horizontal circulatory flow is set up there in the lower region of the absorption zone.